What Is a Coordination Compound? Meaning with Simple Examples
Chemistry · Coordination Compounds · NEET
A coordination compound is a compound where a central metal atom or ion is joined to a group of ions or molecules (called ligands) by sharing electrons. This central metal plus its ligands sits inside square brackets and is called the complex, for example [Co(NH3)6]Cl3. Memory hook: think of the metal as a "boss" and the ligands as "workers" holding its hands inside the brackets, while the ions written outside the brackets are just onlookers.
In [Co(NH3)6]Cl3, the Co3+ ion and its six NH3 ligands stay locked together inside the square brackets (the coordination sphere / complex ion). The 3 Cl- written outside the brackets are counter ions that break free in water. Only these outside ions conduct current or form AgCl.
Your doubts, answered
What is a coordination compound in the simplest words?
It is a compound where a central metal atom or ion is surrounded by, and bonded to, several ions or neutral molecules called ligands. The metal accepts electron pairs that the ligands donate. NCERT says: transition metals bind to anions or neutral molecules by sharing of electrons, and such compounds are called coordination compounds. Example: in [Co(NH3)6]Cl3, one Co3+ ion is bonded to six NH3 molecules.
What do the square brackets in a formula like [Co(NH3)6]Cl3 mean?
The square brackets hold the coordination sphere. Everything inside the brackets (the metal + its ligands) is one unit that stays together in water. In [Co(NH3)6]Cl3, the part [Co(NH3)6]3+ stays as one complex ion. The 3 Cl- written outside the brackets are free ions that break away in water. So brackets = tightly bonded, outside brackets = free.
Is a complex the same thing as a coordination compound?
Almost, but be careful. The complex is the ion or unit inside the brackets, like [Co(NH3)6]3+. The full coordination compound is the complex plus the ions outside the brackets, like [Co(NH3)6]Cl3. So the complex ion is a part; the coordination compound is the whole salt. NEET questions often use both words to mean the same overall compound, so read the formula.
How is a coordination compound different from a double salt?
Both are addition compounds, but they behave differently in water. A double salt (like potash alum KAl(SO4)2.12H2O) breaks completely into all its simple ions (K+, Al3+, SO4 2-) and loses its identity. A coordination compound keeps a stable complex ion in water that does NOT fully break apart. This single point of difference is directly tested by NEET.
Why do CoCl3.6NH3 and [Co(NH3)6]Cl3 mean the same thing?
CoCl3.6NH3 is the old 'addition compound' way of writing it (just adding the pieces). [Co(NH3)6]Cl3 is the modern coordination formula that shows the real structure: the 6 NH3 are bonded to Co inside the brackets, and the 3 Cl- are outside. Werner discovered that the second form is the true picture. NEET expects you to convert the old form into the bracket form.
Why is knowing coordination compounds important for NEET?
It is a full chapter and gives easy, repeated marks. Almost every year NEET asks how many ions or chloride ions a complex gives in water (conductance, AgCl test), complex vs double salt, naming, isomerism, and magnetic moment. If you master the basic meaning first, the harder topics like crystal field theory become much easier.
⚠️ The NEET trap ✗ Counting all chlorine atoms in [Co(NH3)6]Cl3 as free, or thinking every Cl in a complex gives AgCl / conducts current. ✓ Only ions written OUTSIDE the square brackets are free in water. In [Co(NH3)6]Cl3, only the 3 Cl- outside are free (gives 3 AgCl, 4 ions total). In [Co(NH3)3Cl3], all 3 Cl are inside the bracket bonded to Co, so it is neutral, gives 0 free ions and 0 AgCl. 🧠 Inside the brackets = locked (no free ion). Outside the brackets = loose (free ion). Count only the loose ones.
Real NEET questions
NEET 2017
The correct order of the stoichiometries of AgCl formed when AgNO3 in excess is treated with the complexes CoCl3.6NH3, CoCl3.5NH3, CoCl3.4NH3 respectively is:
A · 1 AgCl, 3 AgCl, 2 AgCl
B · 3 AgCl, 1 AgCl, 2 AgCl
C · 3 AgCl, 2 AgCl, 1 AgCl ✓
D · 2 AgCl, 3 AgCl, 1 AgCl
Solution: Only Cl- outside the square brackets is free and precipitates with Ag+ as AgCl. Write each in true coordination form: CoCl3.6NH3 = [Co(NH3)6]Cl3 -> 3 free Cl- -> 3 AgCl. CoCl3.5NH3 = [Co(NH3)5Cl]Cl2 -> 2 free Cl- -> 2 AgCl. CoCl3.4NH3 = [Co(NH3)4Cl2]Cl -> 1 free Cl- -> 1 AgCl. So the order is 3, 2, 1 AgCl, option (C).
NEET 2023 Phase 1
Which of the following forms a set of a complex and a double salt, respectively?
A · K2PtCl2.2NH3 and KAl(SO4)2.12H2O ✓
B · NiCl2.6H2O and NiCl2(H2O)4
C · CuSO4.5H2O and CuCl2.4NH3
D · PtCl2.2NH3 and PtCl4.2HCl
Solution: A double salt breaks fully in water into all its simple ions and loses its identity; a complex keeps a stable complex ion in water. KAl(SO4)2.12H2O (potash alum) gives K+, Al3+ and SO4 2- separately, so it is the double salt. K2PtCl2.2NH3 contains a stable Pt-based complex ion, so it is the coordination compound. Complex + double salt = option (A).
ReNEET 2026
A 1:3 electrolyte in aqueous solution is:
A · [CoCl2(NH3)4]Cl
B · [CoCl(NH3)5]Cl2
C · [Co(NH3)6]Cl3 ✓
D · [Co(NH3)3(NO2)3]
Solution: A 1:3 electrolyte gives one cation and three anions. Only ions outside the brackets are free. [Co(NH3)6]Cl3 -> one [Co(NH3)6]3+ cation and three free Cl- = 1:3. The others give 1:1, 1:2, or are neutral non-electrolytes. Answer (C).
Solved Coordination Compounds NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the central atom in a coordination compound?
It is the metal atom or ion (usually a transition metal like Co, Fe, Ni, Cu) that sits in the centre and accepts electron pairs from the ligands. In [Co(NH3)6]Cl3 the central atom is Co3+.
What is a ligand in one line?
A ligand is an ion or neutral molecule that donates a lone pair of electrons to the central metal and bonds to it. Examples: NH3, H2O, Cl-, CN-.
Are all chlorides in a complex free ions?
No. Only chlorides written outside the square brackets are free in water. Chlorides bonded to the metal inside the brackets are not free and do not give AgCl or conduct current.
Give one everyday or NEET example of a coordination compound.
[Co(NH3)6]Cl3 is the classic NEET example. Others include K4[Fe(CN)6] and [Ni(CO)4]. Chlorophyll and haemoglobin are natural coordination compounds too.
Is a coordination compound the same as a molecular compound?
Not exactly. A coordination compound has coordinate (dative) bonds where the ligand donates both electrons of the shared pair to the metal, and the metal + ligands form a distinct complex sphere. This is why it behaves as one unit in water.
